Method of making high-strength cold-drawn wire



United States Patent 3,502,513 METHOD OF MAKING HIGH-STRENGTH COLD-DRAWN WIRE Lynwood C. Rice, Hudson, and Gordon T. Spare, Chardon, Ohio, assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Oct. 23, 1967, Ser. No. 677,033 Int. Cl. C21d 1/46, 1/18 U.S. Cl. 148-421 8 Claims ABSTRACT OF THE DISCLOSURE A method of producing high-strength, cold-drawn Wire of steel having up to 0.12% carbon, 02 to 1.25% manganese, 0:15 to 0.75% silicon, 0.25 to 0.55% copper, 0.3 to 1.25% chromium, up to 0.65% nickel and up to 0.1% vanadium by pickling rod of the aforementioned composition to remove scale, heating the rod to austenitizing temperature in a non-oxidizing atmosphere, quenching the rod in a molten cascading salt at a temperature in the range of 400 to 600 F., tempering said rod in a molten cascading salt at a temperature in the range of 750 to 950 F., rinsing to remove a substantial portion of said salt, and subsequently cold-drawing said rod to desired gauge.

This invention relates to a method of producing highstrength, cold-drawn steel wire of high ductility. More particularly, the invention relates to a method of producing high-quality, cold-drawn wire of a steel containing about 0.12% carbon, 0.2 to 1.25% manganese, 0.15 to 0.75% silicon, 0.25 to 0.55% copper, 0.3 to 1.25% chromium, up to 0.65% nickel and up to 0.1% vanadium.

Steel of the composition described above has been widely used in architectural applications because of its improved corrosion resistance and the uniform and pleasing color it takes on as it weathers. However, wire applications for this steel have thus far been limited to the making of fasteners for such uses as the architectural application described above. The wire used for such fasteners is lightly drafted from hot-rolled rod and has a strength of only about 100 K Si. or less. More severe drawing of the hot-rolled rod permits a small improvement, that is up to perhaps 130 K s.i. However, if steel of the aforementioned composition could be drawn into wires of 200 to 250 K s.i., the product would become useful for making strand, rope and springs for many other applications.

The patenting heat treatments customarily used to strengthen rope and spring wires before drawing are not suitable for the steel of the aforementioned composition. The conventional patenting treatments require too slow a cooling and ordinary lead patenting is conducted at too high a temperature to produce a satisfactory highstrength structure in the steel. Moreover, although pearlite forms with relative slowness, ferrite precipitates with great speed in this steel.

Water quenching before drawing to form an essentially.

low-carbon martensite structure is a possibility, but it has certain inherent disadvantages. Experience with lowcarbon martensitic wires has shown that it is advisable to temper them before coiling. Quenching and coiling stresses developed, however, can be very high and have in some cases caused the wire to break upon coiling. Tempering to alleviate the stress conditions involves a great waste of heat if the wire must be heated from water cooling temperatures. This is so because the martensite reaction is essentially complete in the steel at relatively high temperatures on the order of 600 or 700 F. Tempering in lead to avoid the aforementioned problem causes the lead to adhere to the wire to some degree, and this is detrimental in drawing and also results in a streaked appearance after drawing.

It has been discovered that the aforementioned difficulty can be overcome by quenching the heated steel of the above-described composition as wire or rod in a molten salt, such as a heat transfer salt, which is cascaded to give an immediate quench with no opportunity for transformation to structure other than martensite. The wire is then tempered in a cascade of molten salt, which may be the same salt, at a somewhat higher temperature. By this practice, it was unexpectedly discovered that the soft, undesirable ferrite structure can be suppressed somewhat more positively with the salt quenching than with the water quench. Since water has a surface heat transfer coefficient (H value) higher than molten salt, it would be expected that water would be preferable. We have discovered that when the steel is taken from a smaller section as a wire or rod, the molten salt is unexpectedly superior in this respect. The differences or discrepancies between water or molten salt can be explained on the basis of the small wire sections and on the peculiar nature of the I-T diagram for steel of this composition. A relatively sluggish A F+C reaction is preceded by a very rapid A F reaction with ferrite beginning to form in less than 2 seconds at 1400 F. If the first relatively ineffective stage of cooling, characterized by slowly collapsing bubbles, lasts a little longer with water than with molten salt, then some ferrite would form at temperatures (near 1400 F.) in water quenching. The second stage of cooling with the salt quenching begins slightly earlier than the water quenching, and prevents the high-temperature ferrite formation. It also is rapid enough to, prevent low-temperature ferrite formation in sections as small as A-inch wires or rods.

The cascading salt quenched and tempered steel wire or rod has a very-high ductility and the high ductility persists even after severe cold reductions. A light scale having a glossy brownish-black color is left on the wire and it creates a pleasing appearance. This scale also imparts good drawing characteristics to the rod when a pro-cleaned rod is hardened by the practice in accordance with the invention. This is partially due to the action of the scale as a lubricant during drawing. By practicing the process described herein in accordance with the invention, tensile strengths in excess of 300 K s.i. can be developed.

In a. presently preferred practice of the invention, rod on the order of Ai-inch diameter is pickled, for example in muriatic acid, rinsed and dried. It is then heated continuously in a reducing atmosphere to austenitizing temperature. Following heating to austenitizing temperature, it is quenched continuously in a molten cascading salt at 400 to 600 F., preferably 450 to 550 F., and then tempered in a salt cascade, which may be of the same salt composition, at 750 to 950 F., preferably 750 to 850 F. After continuous light rinsing, it is coiled and permitted to air cool, followed by baking for several hours at elevated temperature, e.g. 500 F., to drive off hydrogen absorbed during heating. A coating may be applied to the wire (without pickling so as to avoid damaging the martensitic structure) before drawing by conventional methods.

In a specific example, rod of the composition shown in Table I was pickled in muriatic acid, rinsed and dried. It was then heated to 1700 F. in cracked ammonia for 2 /2 minutes, then quenched continuously in cascading salt (sodium nitrate-sodium nitrite eutectic) at 500 F. After quenching, the rod was tempered in a cascade of the same salt at 800 F. Following light rinsing, it was coiled and air cooled, then rinsed and baked for several hours at 500 F. to drive off hydrogen absorbed during heating. A protective lime coating was applied and then the rod was drawn to fine wire by conventional methods.

TABLE I Chemical composition of fit-inch steel wire rod Percent C 0.09 Mn 0.41 P 0.11 S 0.020 Si 0.44 Ni 0.26 Cr 0.73 Cu 0.35

The increase in strength caused by drawing after treating in accordance with the invention is shown in Table II.

TABLE II.EFFECT OF DRAWING ON STRENGTH AND DUCTILITY OF HEAT-TREATED M-INCH STEEL WIRE ROD Cumulative Tensile Reduction of Diameter, reduction, strength, area ductility, inches percent K 5.1. percent The difference in ferrite formation by quenching in different mediums is described in Table III. As can be seen, quenching in water generally results in some ferrite formation near the surface whereas quenching in molten al doe o TABLE III.-HEAT TREATlIJlET OF %-INCH STEEL WIRE [Samples pickled in muriatic acid, rinsed and dried. Heated in 1700 F cracked NH3 for 2% minutes] Estimated ferrite It is apparent from the above that various changes and modifications may be made without departing from the invention; therefore, the scope of the invention should be limited only by the appended claims wherein what is claimed is:

1. A method of producing high-strength, high-ductility, cold-drawn wire of steel having up to 0.12% carbon, 0.2 to 1.25% manganese, 0.15 to 0.75% silicon, 0.25 to 0.55% copper, 0.3 to 1.25% chromium, up to 0.65% nickel and up to 0.1% vanadium, comprising pickling rod of the aforementioned composition to remove scale, heating the rod to austenitizing temperature in a non-oxidizing atmosphere, quenching the rod in a molten cascading salt at a temperature in the range of 400 to 600 F., tempering said rod in a molten cascading salt at a temperature in the range of 750 to 950 F., rinsing to remove a substantial portion of said salt, and subsequently colddrawing said rod to desired gauge.

2. A method according to claim 1 wherein said quenching is to a temperature in the range of 450 to 550 F.

3. A method according to claim 1 wherein said tempering is at a temperature in the range of 750 to 850 F.

4. A method in accordance with claim 1 wherein prior to said cold-drawing, said rod is baked for up to 6 hours at an elevated temperature to remove hydrogen from the surface thereof.

5. A method in accordance with claim 4. wherein said baking for hydrogen removal is performed from 1 to 6 hours at a temperature of 400 to 600 F.

6. A method in accordance with claim 1 wherein prior to said cold-drawing, the rod is coated.

7. A method in accordance with claim 6 wherein prior to said cold-drawing, said rod is coated with a coating from the group consisting of lime and borax.

8. A method in accordance with claim 1 wherein said molten salt contains sodium nitrate.

References Cited UNITED STATES PATENTS 1,924,099 8/1933 Bain et al 148l43 HYLAND BIZOT, Primary Examiner W. W. STALLARD, Assistant Examiner 

